Scale Keyframes Together From "Medium Point"
In the Dope Sheet, the position of the 2D cursor (the blue line) will be the pivot point:

As  atomicbezierslinger says, if you switch to the Graph Editor (CtrlShiftTab) you'll have the choice between Bounding Box Center, 2D Cursor and Individual Centers:

if you want the middle of your selected keyframes to be the pivot point, choose Bounding Box Center: